1. Field of the Invention
The present invention relates to architectures and designs of mitigation techniques for RF (radio frequency) systems against strong RF interferences.
Strong leakage signals from nearby transmitters often saturate receiver front-ends of RF systems. The invention relates to (1) mitigation architectures and designs of self-jamming in antenna farms where transmitting (Tx) and receiving (Rx) apertures are adjacent to one another over limited space, such as cell phone base-stations, RF repeaters, mobile communications terminals, FMCW (frequency modulated continuous-wave) radars, RF systems on air mobile platforms, RF systems on ships, RF systems on satellites, and etc.
On the other hand, strong external jamming signals often reduce sensitivity of RF receivers. This invention also relates to (2) mitigation architectures and designs of sensitive RF receivers against strong external jamming systems. In particular, this invention will enable airborne GPS (global positioning system) receivers to function properly, circumventing potential front end saturations due to strong external jamming signals.
2. Description of Related Art
In the field of radio frequency engineering, interference is not hard to find. In fact, it is actually difficult to avoid. By definition, interference originates from a source external to a signal path and produces undesired artifacts in the signal. Deliberate use of an interfering signal to disrupt communications is frequently referred to as jamming. A radio frequency, or RF, is loosely defined as being in that portion of the electromagnetic spectrum above audio (about 20 kHz) but below infrared (about 30 THz).
Sources of RF interference fall into two broad categories: intentional and unintentional interference. The purpose of intentional radio jamming is to make reception of desired signals difficult or impossible. One method that jamming is accomplished is by transmitting a signal on the same frequency as that used for communications. Consequently, jamming signals could be used to block multiple frequencies as well. On the other hand, power jammers feature very strong signals to saturate front-ends (the transmitting side) of communications, radiometers, or radar receivers, producing high intermodulation (IM) noises over receiving bandwidths, thus blocking communications signals.
GPS (global positioning system) receivers are effective in extracting the navigational information signals transmitted from satellites due to a large processing gain with spread spectrum techniques. Nevertheless, performance will significantly degrade if any strong interference source coexists with the information signal. Typically, a jamming signal with a power level less than 40 dB with respect to the signal power level, i.e. jammer-to-signal ratio (JSR) of 40 dB, can be tolerated in a GPS receiver. In practice, the received GPS satellite signal is about −160 dBm on the ground, and is too weak to tolerate any existence of easily generated strong unintentional RF interference and intentional jammers. Consequently, a technique for powerful jammer suppression has gained much attention.
A communication system can be full duplex or half duplex. In a full duplex system communicating transceivers can receive and transmit signals simultaneously, through different frequency bands. This is known as frequency division. Most current cellular standards have adopted the use of full duplex communications. However, a full duplex receiver is susceptible to a problem called “self-interference”. Typically, full duplex transceivers resolve the self-interference problem by suppressing signals in the transmit frequency band at the receiver input.
Unintentional jamming may result from cohabitation of multiple RF functions in an instrument over a small volume and operating concurrently in nearby frequency spectra. Some examples include multifunction handheld devices with GPS [1], or devices with both Will and Bluetooth [2]. WiFi is the current embodiment of wireless internet, while Bluetooth is a branch of wireless technology aimed at short distance data transmission (i.e. between a wireless headset and a cellular phone). These unintentional jammers, referred to as electromagnetic interferences (EMI), are key concerns for their designs.
U.S. Pat. No. 7,155,179[1] describes a full duplex transceiver having a method for immunizing itself against self-jamming. It uses a centre frequency of jamming signals to down-convert the desired signal. This system works for single antenna element systems and requires significant dynamic range up to the high pass filter.
US Patent application #20100022201 [2] describes a full duplex transceiver using MIMO (multiple input and multiple output) systems to avoid self-jamming.
For some advanced radars such as FMCW radars [3], it is desirable to co-locate separated Tx and Rx apertures allowing “full duplex” operation between Tx and Rx functions, thus improving the radar operational duty cycle. Mitigations of self-jamming effects due to the leakage from nearby Tx apertures are one of the current concerns in radar designs.